The invention relates to the generation of three-dimensional objects by imaging, a field also known as stereolithography, and in particular to thermal stereolithography of various objects, including flexographic printing plates.
The generation of three dimensional (3D) objects by imaging a liquid resin is well known and has been commercially available for many years. Typically the liquid resin is made to polymerize in areas exposed to intense UV light from a laser or a mask illuminated by a UV lamp. The two best known applications are building 3D models by a process known as stereolithography and manufacturing flexographic printing plates. Flexographic printing plates are printing plates having considerable surface relief.
Previous methods for creating a 3D object by imaging a liquid resin use a photonic principle. Such processes fall under what is known as the xe2x80x9cLaw of Reciprocityxe2x80x9d. This law states that imaging for a long time using a low intensity light will give the same result as imaging for a short time using a high intensity light, as long as the exposure (defined as the integral of the light intensity over time) stays the same. A different way to state this behaviour is to say that the exposure process falls under the law of linear superposition. The law of linear superposition states that: f(a+b)=f(a)+f(b). Simply stated, the combined exposure (a+b) yields the same result as exposure xe2x80x9caxe2x80x9d followed by exposure xe2x80x9cbxe2x80x9d. There are some polymerization processes which deviate from the xe2x80x9cLaw of Reciprocityxe2x80x9d such as two-photon absorption processes, in which the rate is proportional to the square of the intensity. Such processes still integrate light and suffer from very low sensitivity requiring high amounts of UV light.
Because of this behaviour, it is not possible to focus an exposure deep inside a liquid resin without also exposing the volume above the desired exposure point. This is shown in FIG. 1. When beam 3 is focused by lens 4 to a point 5 inside liquid resin 1 to polymerize resin 1 at point 5, the area above point 5 will undergo polymerization as well. As point 5 moves along a line inside the liquid resin, points in the immediate vicinity of the line along which point 5 moves, the resin are subject to intense exposure for a short time. The volume 6 above the line through which the exposing light passes before reaching point 5 is subjected to a weak exposure for a longer time (due to the large overlap of the beams forming point 5). Since the product of intensity and exposure is about the same in volume 6 as it is along line 5, volume 6 will polymerize as well. If the absorbance of material 1 is high, volume 6 will actually receive a higher exposure than the desired area along the line travelled by point 5 as point 5 is scanned to cover a large area. Volumes deeper in the fluid 1 than point 5 will be exposed as well. These volumes will receive lower exposures since part of the light is absorbed before it reaches those volumes.
For these reasons, prior art systems can only expose the top layer of a liquid polymer and require elaborate means to lower the polymerized layer and keep it submerged, in order to build an object layer-by-layer, always exposing only the top layer.
A new class of material known as thermosensitive, or thermal, materials has become available. Some thermosensitive materials solidify upon heating to a temperature in excess of a threshold temperature. Thermosensitive, or thermal, materials include both polymerizable materials (xe2x80x9cresinsxe2x80x9d) and coalescent materials. Thermosensitive coalescent materials typically comprise small particles which coalesce upon the material reaching a threshold temperature. At temperatures below the threshold thermosensitive materials remain fluid. Because of this property thermosensitive materials operate completely outside the xe2x80x9cLaw of Reciprocityxe2x80x9d or the principle of linear superposition. An example of a thermosensitive process is melting. A block of lead can be melted by heating it up to 500xc2x0 C. but cannot be melted by heating it up twice to 250xc2x0 C. If kept at 250xc2x0 C. for even a long time the lead block will remain solid. This non-integrating behavior is typical of all thermosensitive materials.
Methods have been known since the 1960""s for making printing plates involving the use of imaging elements that utilize heat-driven processes rather than photosensitivity. U.S. Pat. No. 3,476,937, Vrancken, discloses a process for making printing plates by imaging particles of thermoplastic polymer in a hydrophilic binder. The particles coalesce under the influence of heat, or heat and pressure. This process is used in heat-based lithographic plates that are developed using various aqueous media. U.S. Pat. No. 3,793,025, Vrancken, discloses the addition of a pigment or dye to a thermosensitive material in the process of Vrancken ""937. The pigment or dye converts visible light to heat. U.S. Pat. No. 4,004,924, Vrancken, further discloses the use of hydrophobic thermoplastic polymer particles in a hydrophilic binder together with a light-to-heat converter. In Vrancken ""924, the combination is employed specifically to generate printing masters by flash exposure. Various systems for using thermal coalescing materials to make lithographic printing plates are known. One example of a thermal coalescing material used for these purposes is Thermolite(trademark) available from Agfa of Mortsel, Belgium.
Thermal coalescent materials typically comprise a suspension or latex of particles which coalesce to form a larger solid mass upon heating. Typically, coalescent materials comprise a suspension of uncoalesced hydrophobic thermoplastic polymer particles mixed with a component which converts electromagnetic radiation to heat. Macroscopically, coalescent materials appear as a liquid which solidifies locally upon being heated beyond a threshold temperature.
Some prior art processes use laser heating for stereolithography by cutting thin sheets or melting a thin layer of powder. However, neither process is suitable for true 3D imaging as the materials used will scatter the light. These processes it can only be used to form objects in thin layers. Furthermore, in these processes the material starts off as a solid and the heat turns it into a liquid or gas.
This invention exploits the fact that the exposure of thermosensitive materials does not obey the law of superposition or the law of linear superposition. Three-dimensional objects are created inside a volume of liquid thermosensitive material by 3D scanning of the volume using a focussed light beam, preferably in the IR part of the spectrum. The focussed light beam heats the thermosensitive material to a high temperature in the immediate vicinity of the focal point. The thermosensitive material solidifies rapidly at the points the light is focussed, due to the high temperature, but heats up only slightly in all other areas. As the beam is scanned the areas where temperatures have not reached threshold of the thermosensitive material cool down. The exposure is not integrated in these areas. The unexposed parts of the thermosensitive material may be heated repeatedly to temperatures lower than the threshold temperature without solidifying.
The efficiency of the process can be further increased by providing multiple beams, from different directions, which converge on a common point. This also allows nearly constant exposure through the volume of the thermosensitive material.